1. Technical Field
The present invention relates to a wavelength locker module for detecting quantities of light beams branched from an optical signal for optical communication, the quantities being utilized for detection of a wavelength variation in the optical signal, and relates to a wavelength controller, including a wavelength locker module, for detecting and suppressing a wavelength variation in an optical signal.
2. Related Art
For optical communication, a wavelength division multiplexing (WDM) system, for instance, is utilized which simultaneously transmits multiplexed laser beams of different wavelengths having information superposed thereon. To realize a stable large-capacitance optical transmission, the channel spacing, i.e., wavelength interval between adjacent laser beams, is narrow and the wavelength of each laser beam is stabilized. The demand for wavelength stabilization intensifies in a dense wavelength division multiplexing (DWDM) system where the channel spacing is made smaller for larger transmission capacity.
A light source for the WDM system is comprised of, e.g., a distributed feed-back (DFB) semiconductive laser which is excellent in single-mode oscillation but generates output light whose wavelength tends to vary with a temperature change and secular change in the DFB semiconductive laser. For wavelength stabilization of the output light of the DFB semiconductive laser, therefore, a wavelength controller with a so-called wavelength locker module is employed.
A typical wavelength controller is designed to detect a wavelength variation (wavelength shift) in incident light from a semiconductive laser, and make the wavelength of laser light constant by changing the temperature of the semiconductive laser so as to compensate for the wavelength variation based on a temperature-wavelength characteristic of the semiconductive laser. Since an intensity level variation in the incident light caused by a secular change of the semiconductive laser produces an error in wavelength variation detection, the wavelength controller uses the wavelength locker module for branching the incident light into two or more branched light beams and for detecting light beam quantities, whereby the intensity level variation and the wavelength variation in the incident light are detected independently of each other based on the detected light beam quantities.
In some wavelength locker modules, an optical branching coupler is used to branch incident light. This makes it difficult to attain a compact wavelength locker module because of the presence of a large-sized branching coupler. Thus, another type of wavelength locker module has been employed, which is provided with a filter for branching or demultiplexing incident light instead of a branching coupler.
FIG. 10 shows by way of example a conventional wavelength locker module with a branching filter. In this wavelength locker module 8, the output light of a semiconductive laser enters a branching filter 13 through an incident section 1 and a lens 2. The branching filter 13 is comprised of a bandpass filter, an etalon or the like, and serves to reflect light, falling within a prescribed wavelength region, at a constant reflection coefficient. The incident light entering the branching filter 13 is reflected by the branching filter 13 at the constant reflection coefficient, if the wavelength falls within the prescribed wavelength region. Part of the incident beam is reflected by the branching filter 13 and enters a second photodiode 5, which is provided at its incident side with a condensing lens 10. The remaining incident transmits through the branching filter 13 and enters, via a wavelength selective transmission filter 6, a first photodiode 4 which has an incident side provided with a condensing lens 9.
The wavelength selective transmission filter 6 has the transmittance-wavelength characteristic exemplarily shown by the characteristic curve b in FIG. 9 in which the transmittance of the transmission filter 6 is plotted in the right ordinate axis and the output of the first photodiode 4 is plotted in the left ordinate axis, with the wavelength of incident light entering the filter 6 plotted in the abscissa axis. As apparent from FIG. 9, the transmittance of the filter 6 and the output of the first photodiode 4 vary in dependence on incident light wavelength. Specifically, when the incident light wavelength shifts toward the long-wavelength side from the rated wavelength A to the wavelength B, the output of the first photodiode 4 changes to decrease by xcex94b from the value a, whereas the photodiode output changes to increase by xcex94c from the value a when the wavelength shifts toward the short-wavelength side from the rated wavelength A to the wavelength C.
As mentioned above, there occurs a change in the output of the first photodiode 4 with a wavelength variation in the semiconductive laser light. In addition, an intensity variation in the laser light caused by a secular change in the semiconductive laser can cause a variation in the output of the first photodiode 4. In the wavelength locker module 8 shown in FIG. 10, the intensity of the incident light entering the wavelength locker module 8, which intensity corresponds to the laser output intensity, is detected by a second photodiode 5. Then, it is determined that a wavelength variation occurs in the laser light, if the output of the second photodiode 5 has the ordinary value and if the output of the first photodiode 4 shifts from the ordinary value.
FIG. 11 shows a conventional wavelength locker module having a demultiplexing filter used to demultiplex incident light for the detection of wavelength variation. The demultiplexing filter 19 is comprised of a bandpass filter, an etalon or the like and reflects incident light, falling within the prescribed wavelength region, at the reflection coefficient that varies depending on the wavelength of the incident light. In the wavelength locker module 8, the output light of a semiconductive laser enters the demultiplexing filter 19 through an incident section 1 and a lens 2, and part of the incident light passes through the demultiplexing filter 19 to enter a first photodiode 4, whereas the remaining incident light is reflected by the demultiplexing filter 19, to be entered into a second photodiode 5. The output characteristics of the first and second photodiodes 4, 5 are shown by the characteristic lines g and h in FIG. 13. As shown in FIG. 13, the outputs of the photodiodes 4, 5 vary depending on the wavelength of the incident light entering the demultiplexing filter 19, i.e., the wavelength of the output light of the semiconductive laser. Thus, it is possible to detect, based on the variations in outputs of these photodiodes, variations in the intensity level of the incident light caused by a wavelength variation of the incident light and caused by a secular change in the semiconductive laser.
Although the wavelength locker modules of the kinds exemplarily shown in FIGS. 10 and 11 are compact as compared to the wavelength locker module including an optical branching coupler, they require an increased incident angle of the incident light entering the branching filter 13 or the demultiplexing filter 19. For this reason, the filter transmission characteristic, i.e., the photodiode output characteristic is different between when the incident light to the filter is a horizontally polarized wave (shown by the characteristic line P in FIG. 12) and when it is a vertically polarized wave (shown by the characteristic line S in FIG. 12). In the case of the filter whose light transmittance has a polarization dependency to produce a substantial polarization-dependent loss, an error occurs in the wavelength variation detection by the wavelength locker module.
An object of the present invention is to provide a wavelength locker module which is compact and capable of accurately detecting quantities of light beams branched from incident light, thereby improving the accuracy of detection of wavelength variation in the incident light based on the light beam quantities.
Another object of the present invention is to provide a wavelength controller which is capable of detecting and suppressing a wavelength variation in incident light based on light beam quantities detected by a wavelength locker module provided therein.
A wavelength locker module according to one aspect of the present invention comprises a prism for dividing incident light into at least first and second branched light beams; a wavelength selective filter for permitting part of the first branched light beam emitted from said prism to pass therethrough; a first light quantity detector for receiving the part of the first branched light beam having passed through said wavelength selective filter; and a second light quantity detector for directly receiving the second branched light beam emitted from said prism.
The wavelength locker module of this invention which employs the prism as an element for branching incident light makes it possible to appropriately divide the incident light into branched light beams even when the incident light enters the prism at a small angle. A small incident angle decreases an error, caused by polarization-dependent loss, in detecting a wavelength variation, permitting an accurate wavelength variation detection. In addition, the prism is compact in size as compared to an optical branching coupler, contributing to the realization of a compact wavelength locker module.
According to another aspect of the present invention, there is provided a wavelength controller which comprises a wavelength locker module of the aforementioned type; wavelength variation detecting means for detecting, based on outputs of the first and second light quantity detectors of the wavelength locker module, a wavelength variation in incident light entering the wavelength locker module; and wavelength variation suppressing means for suppressing the wavelength variation in accordance with a detection result obtained by said wavelength variation detecting means.
With this invention, since the wavelength locker module is compact, the wavelength controller can be compact in size. It is also possible to accurately detect and suppress the wavelength variation of the incident light based on the outputs of the light quantity detectors, thereby maintaining an optical signal constant, which signal is generated by an optical signal generator such as a semiconductive laser for use with the wavelength controller of this invention.